Any specific layer in a multi-layer protocol stack. The layer above is called the (N+1)-layer, the layer below is called the (N-1)-layer. This notation is also used in the model for other concepts that are related to these layers, for example, (N)-protocol, (N+1)-service.

Functions related to the management of the (N)-layer partly performed in the (N)-layer itself according to the (N)-protocol of the layer (activities such as activation and error control) and partly performed as a subset of systems management.